Conceptually, PEGylation has been described as the attachment of a poly(ethylene glycol) derivative to a pharmacologically active agent to thereby form a “conjugate.” In practice, a polymeric reagent (which is a water-soluble polymer bearing a reactive functional group or an “activated” functional group) is typically reacted with an active agent of interest in order to attach or link the water-soluble polymer (either directly or through a linking moiety) to the active agent through a covalent bond. As compared to the active agent lacking attachment of the water-soluble polymer, the conjugate may possess an extended half-life in vivo, decreased immunogenicity, increased hydrophilic character, or some combination of the foregoing. Harris et al. have provided a review of the effects of PEGylation on pharmaceuticals. Harris et al. (2003) Nat. Rev. Drug Discov. 2(3):214-221.
Several examples of PEGylated active agents available commercially include PEGASYS® PEGylated interferon alpha-2a (Hoffmann-La Roche, Nutley, N.J.), PEG-INTRON® PEGylated interferon alpha-2b (Schering Corp., Kennilworth, N.J.), NEULASTA™ PEG-filgrastim (Amgen Inc., Thousand Oaks, Calif.), and MACUGEN® PEGylated aptamer (Pfizer Inc., New York, N.Y.). Although each of the active agents in each of these examples is a “large molecule,” small molecules such as distearoylphosphatidylethanolamine (Zalipsky (1993) Bioconjug. Chem. 4(4):296-299) and fluorouracil (Ouchi et al. (1992) Drug Des. Discov. 2(1):93-105) have also been PEGylated. Thus, many types of molecules can potentially benefit from PEGylation.
While the general benefits of PEGylation are known, attaching a poly(ethylene glycol) derivative to an active agent is often challenging and may not always be possible. For example, difficulties can be encountered when the active agent of interest does not include a chemical functional group suitable for reaction with a polymeric reagent. Further, to the extent that a suitable chemical functional group is present on the active agent of interest, the resulting conjugate may be insufficiently pharmacologically active as a result of the attached polymer interfering with, for example, a binding site necessary for activity of the active agent.
U.S. Patent Application Publication No. 2005/0136031 describes, among other things, a conjugate of poly(ethylene glycol) and a narcotic antagonist. In order to effect conjugation at the desired location, however, several steps must be taken. As described in this reference, the 6-keto group of 3-MEM-naloxone (a 3-hydroxy-protected naloxone) is reduced with sodium borohydride (NaBH4) to form an α- and β-epimer mixture of 6-hydroxy-3-MEM-naloxol. A polymeric reagent is thereafter covalently attached at the available hydroxyl group to thereby form an α- and β-epimer mixture of 6-polymer-3-MEM-naloxol. Once the protecting group is removed, α- and β-epimers are separated and isolated using an appropriate column. Separation and isolation of epimers is desired because, as shown in U.S. Patent Application Publication No. 2005/0136031, individual isomers of 6-polymer-3-MEM-naloxol have different properties.
Stereoselective reduction of naltrexone to α-naltrexol using tri-sec-butylborohydride has been described by Malspeis et al. Malspeis et al. (1975) Res. Commum. Chem. Pathol. Pharmacol. 12(1):43-65. Malspeis et al., however, does not describe stereoselective reduction of compounds other than naltrexone.
Although the approach for preparing compositions comprising substantially pure isomers of 6-polymer-3-MEM-naloxol described in U.S. Patent Application Publication No. 2005/0136031 is effective, an approach that requires fewer steps—such as eliminating the need to separate and isolate individual epimers—would be advantageous. Thus, one or more embodiments of the present invention provide, among other things, a synthetic method that eliminates the need to separate and isolate individual epimers of conjugates of poly(ethylene glycol) and a narcotic antagonist.